1. Field of the Invention
This invention relates to a method and apparatus for cutting a stream of viscous molten glass descending from a glass feeder into gobs of optimum size for the forming of various types of glass products such as bottles, tableware and other glass containers and optical lenses.
2. Description of the Prior Art
A process to make such typical glass containers as beer and milk bottles will be briefly described in the following. First, a mixture of ingredients is melted in a melting furnace to produce liquid glass that is then fed into a glass feeder below the furnace. The glass feeder has an orifice through which a continuous stream of viscous molten glass flows downward. Gobs (or lumps of viscous molten glass) are then made by cutting the descending stream of viscous molten glass at given intervals by means of a gob cutter. Each gob falls into a preliminary mold of bottle-making machine waiting below where it is formed into a parison, which, in turn, is then finished into a product bottle in a final mold.
The gob cutter comprises a pair of flat blades, each having a V-shaped cutting edge at the forward end thereof. The descending stream of viscous molten glass is cut into gobs by means of the oppositely disposed blades that are moved horizontally to pass across (and overlap) each other.
The cutting of the viscous molten glass by the blades has conventionally been performed in two broadly classified ways. One of them uses two arms having ends which pivot horizontally, with respect to the axis thereof, along an arched path like the blades of the scissors. At least a pair of facing blades are attached to the other ends of the two arms. Cutting is performed by crossing the opposing blades by moving the arms like the blades of scissors are moved.
The other one uses two horizontal blades fastened to two carriages that are reciprocatably held by a guide member so that the blades face each other. Viscous molten glass is cut into gobs by the linear horizontal motion of the oppositely disposed blades.
In these conventional methods, viscous molten glass is cut halfway in its descending stream by means of the blades that move along either an arched or a linear path in a horizontal plane. In each cutting operation, the two blades, which horizontally move and cross each other, pause twice; first in making a cut and then before moving again after a retreat from the cutting position. These pauses in the reciprocating cutting cycle not only disturb a continuous operation but also cause the vibration of the blade moving mechanism which increases as the moving speed of the blades increases. This barrier limits the operation speed of the horizontal reciprocating blades. With the conventional methods, therefore, the gob cutting and bottle making efficiency cannot be increased beyond a certain limit. A maximum gob cutting speed with one of the most productive methods of the linear horizontal motion type is about 250 pieces per minute.
Shear mark, which is a crease formed in the top and bottom ends of a gob cut from a stream of viscous molten glass, is a serious problem with glass products. As its presence lowers the quality of a product, it is desirable to reduce shear mark to a minimum. Shear mark results from the contact between the blades and viscous molten glass, so that its incidence increases as the duration of contact therebetween increases. As the blades in retreat are cooled, a long contact time provides a favorable condition for the forming of shear mark. To decrease the formation of shear mark, accordingly, it is essential to increase the cutting speed and shorten the contact time between the blades and glass. Because of the mechanical limit described before, however, it has been practically difficult to increase the cutting speed and decrease the formation of shear mark.
A typical example of a conventional linear cutting apparatus is disclosed in U.S. Pat. No. 4,215,611. This apparatus is designed to increase the cutting efficiency by making cutting strokes with the aid of an air cylinder. The apparatus also has a mechanism to tilt one of the paired blades when excess tension works thereon when they overlap each other in a cutting operation. But even this improved linear cutting apparatus is not free from the pauses of the blades. Accordingly, the limit on productivity improvement has remained unremoved and the aforementioned problems unsolved. While necessitating a complex control mechanism, in addition, this improved apparatus has also suffered from limited durability of its component parts.